In the polar RbCs molecule, the strong spin-orbit coupling between the $A\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Sigma}^{+}$ and $b\phantom{\rule{0.2em}{0ex}}^{3}\ensuremath{\Pi}$ diabatic electronic states correlated to the $\mathrm{Rb}(5s)\mathrm{Cs}(6p)$ dissociation limit is at the origin of a global mixing of the two vibrational ${0}^{+}({P}_{1∕2,3∕2})$ series coupled radiatively to both $X\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Sigma}^{+}$ and $a\phantom{\rule{0.2em}{0ex}}^{3}\ensuremath{\Sigma}^{+}$ states. This so-called ``resonant coupling'' plays a crucial role in the formation of ultracold stable RbCs molecules through photoassociation into ${0}^{+}$ levels followed by stabilization through spontaneous emission. We analyze quantitatively the mechanisms of photoassociation and stabilization through ${0}^{+}$ levels, starting from and leading to either the singlet or the triplet states and we compare the efficiency of the four paths leading to the formation of stable RbCs molecules. Comparison between the two isotopomers $^{87}\mathrm{Rb}^{133}\mathrm{Cs}$ and $^{85}\mathrm{Rb}^{133}\mathrm{Cs}$ is also reported: the overall process of formation of stable molecules is one order of magnitude larger for $^{87}\mathrm{Rb}\mathrm{Cs}$. The simple two-channel model presented here yields the general rules for a standard analysis of the effects of resonant coupling between electronic states. To underline the specificity of heteronuclear molecules, the ${0}^{+}$ coupled system of the RbCs molecule is compared to its analog, the ${0}_{u}^{+}$ system of the ${\mathrm{Rb}}_{2}$ molecule.